The invention relates to an X-ray cassette for daylight X-ray systems, composed of:
a flat housing, impervious to light, containing on one side a light-tight, closeable, slit-shaped opening for the insertion of film, PA1 two intensifying screens that have been inserted into the housing substantially parallel to each other, one of which is affixed to an elastic pad and the other to a firm pressure plate that is moveable in a direction substantially perpendicular to the plane of the intensifying screens, PA1 an externally operated apparatus to simultaneously open the light-tight closure of the film insertion opening and lift the pressure plate and its intensifying screen from the opposite intensifying screen or from the sheet of film located between the two intensifying screens.
Such X-ray film cassettes are known from German PS No. 21 66 251, German OS No. 24 38 278 and U.S. Pat. No. 3,870,889. These are used in daylight systems, which make it possible to automatically load and unload X-ray cassettes with film without a darkroom. In this method, the empty cassette with the film insertion slot up is inserted into a film dispenser, which has a device to open the film insertion opening and simultaneously lift the pressure plate and its screen from the opposite intensifying screen so that the sheet of film can fall from the dispenser into the cassette where it comes to rest between the two intensifying screens. When the cassette is removed from the dispenser, the film insertion opening is closed to light and the pressure plate presses the two intensifying screens into good contact with both sides of the X-ray film. After exposure, the cassette is inserted into an unloading apparatus--for example, a developing machine--wherein the film insertion opening is opened again and the pressure plate lifted, so that the film falls of its own weight either directly into a developing machine or into a collector from where it can be transported, as desired, into a developing machine. This type of apparatus is described in U.S. Pat. No. 3,715,087.
During the unloadng procedure, the film can, despite the release of the pressure plate, stick to one or both intensifying screens and either not drop or be delayed. Delays of more than three seconds hinder rapid and continuous work flow. Various measures have already been proposed for faster and more reliable unloading of the cassette.
According to German OS. No. 24 38 278, either a part of the intensifying screen or a special metal part is so linked to the cassette wall that the free ends swing away from the wall to a perpendicular position and thus release the film. The construction requires that the cassette be placed in a specific position for loading as well as unloading. A change in the orientation of the cassette would hinder loading; in unloading, the apparatus would be useless.
European Patent Application EP No. 00 75 876 describes a cassette furnished with a "buckler", which is automatically activated in the unloading process and momentarily pushes against the film edges, buckling the film and thus releasing it from the screens. Although the relatively expensive construction accelerates unloading, the desired short fall time of three seconds is not always attained, especially when intensifying screens with smooth surfaces and large film sizes are used.
In WO No. 84/01040, a lever serves the same purpose. It is installed in a corner of the cassette on the side opposite the opening of the cassette and on being activated, pushes the film in an essentially diagonal direction with respect to the cassette wall, as a result of which the film is buckled and released from the intensifying screens. However, the activation of the lever is not automatic, but requires an extra handle.
For another type of cassette, namely, the so-called book cassette, in which the housing and the cover are connected by a hinge, procedures are described to facilitate the release of the film from the intensifying screen. In X-ray cassettes in accordance with U.S. Pat. No. 3,511,990, both intensifying screens are placed on layers of elastic material, the edges of which bear thicker elastic strips; when the cassette cover is opened, the expansion of the elastic edge strips lifts the film from the intensifying screens. This arrangement cannot be used in the daylight cassettes of the subject invention in which one intensifying screen must be attached directly to the pressure plate without an elastic underpad.
In addition to the above described individual disadvantages, all of these prior art developments have the common disadvantage of requiring additional mechanical devices that complicate cassette production without the required short film drop times being attained.
Therefore the object of the subject invention is to produce an X-ray film cassette of the above named type such that very short film drop times are attained at unloading without additional mechanical devices. An additional object of the invention is to improve the known cassettes furnished with mechanical devices so that film drop times of less than three seconds are attained.